Three-dimensional printing using carbon nanostructures

ABSTRACT

Objects produced by conventional three-dimensional printing methods often have limited structural quality. Printing compositions to address this issue can include a solidifiable matrix and a plurality of carbon nanostructures dispersed in the solidifiable matrix. The carbon nanostructures include a plurality of carbon nanotubes that are branched, crosslinked, and share common walls with one another. Three-dimensional printing methods utilizing such printing compositions can include: depositing the printing composition in a layer-by-layer deposition process, and while depositing the printing composition, applying a focused input of microwave radiation in proximity to a location where the printing composition is being deposited. The focused input of microwave radiation heats the carbon nanostructures at the location and promotes consolidation of the printing composition within an object being produced by the layer-by-layer deposition process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §119from U.S. Provisional Patent Applications 62/010,973 and 62/010,977,each filed on Jun. 11, 2014 and incorporated herein by reference in itsentirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

FIELD

The present disclosure generally relates to three-dimensional printingand, more particularly, to the use of microwave heating in conjunctionwith three-dimensional printing.

BACKGROUND

Three-dimensional (3-D) printing, also known as additive manufacturing,is a rapidly growing technology area that operates by depositing smalldroplets or streams of a melted or solidifiable printing material inprecise deposition locations under control of a computer. Deposition ofthe printing material results in gradual, layer-by-layer buildup of anobject, which can be in any number of complex shapes. Printing materialsthat can be used in three-dimensional printing include polymers andother solidifiable substances.

One of the shortcomings associated with conventional three-dimensionalprinting methods is incomplete fusion of the melted or solidifiableprinting material within the printed object. As used herein, the term“fusion” will refer to the consolidation of a deposited printingcomposition to form a coherent structure. The terms “fusion” and“consolidation” may be used synonymously herein. Incomplete fusion ofthe printing material can be especially prevalent between adjacentlayers of a printed object, which can result in structural weak points.In addition to producing structural weakness, incomplete fusion of theprinting material can convey surface roughness to printed objects on atleast a microscopic level, thereby resulting in a “pixelated” appearanceunder magnification. Presently, there are no effective ways to repair orpost-process printed objects in order to rectify these issues. Incontrast, traditional manufacturing techniques, such as molding and/ormachining, generally form objects that are more homogenous in nature,contain fewer structural weak points, and have a smoother surfacemorphology.

Despite their shortcomings, objects produced by conventionalthree-dimensional printing methods can often be sufficient for rapidprototyping purposes. In this regard, prototypes having a variety ofshapes and sizes can be produced, and tolerances are usually limitedonly by the size of the printer's deposition nozzles. Rapid productionof prototypes represents a significant strength of three-dimensionalprinting, and there is concurrent interest in applying printing methodsfor mass manufacturing. For mass manufacturing purposes, however, theaforementioned defects resulting from incomplete fusion during printingcan problematic, particularly for producing high-performance and/orexacting-tolerance objects with a desired degree of durability andquality. Although incomplete fusion can be mitigated somewhat byreducing the size of the deposition nozzles, albeit at the drawbacks ofincreased printing times and associated higher costs, there is currentlyno way to fully address the issue of poor fusion duringthree-dimensional printing.

In view of the foregoing, increasing the extent of fusion within printedobjects would be of significant interest in the art. The presentdisclosure satisfies the foregoing need and provides related advantagesas well.

SUMMARY

In various embodiments, the present disclosure provides methods forthree-dimensional printing using a printing composition containing asolidifiable matrix and a plurality of carbon nanostructures dispersedin the solidifiable matrix. The carbon nanostructures include aplurality of carbon nanotubes that are branched, crosslinked, and sharecommon walls with one another.

In some embodiments, the present disclosure provides three-dimensionalprinting methods that include depositing a printing composition in alayer-by-layer deposition process, and while depositing the printingcomposition, applying a focused input of microwave radiation inproximity to a location where the printing composition is beingdeposited. The printing composition contains a solidifiable matrix and aplurality of carbon nanostructures dispersed in the solidifiable matrix.The carbon nanostructures include a plurality of carbon nanotubes thatare branched, crosslinked, and share common walls with one another.Applying the focused input of microwave radiation heats the carbonnanostructures at the location and promotes consolidation of theprinting composition within an object being produced by thelayer-by-layer deposition process.

In some embodiments, the present disclosure provides three-dimensionalprinting methods that include depositing a printing composition in alayer-by-layer deposition process in which the printing composition isdeposited from a print head that is coupled to a microwave transmissionline, establishing a standing wave in the microwave transmission line,and while depositing the printing composition, contacting the standingwave at one or more locations with a previously deposited layer of anobject being produced by the layer-by-layer deposition process. Theprinting composition contains a solidifiable matrix and a plurality ofcarbon nanostructures dispersed in the solidifiable matrix. The carbonnanostructures include a plurality of carbon nanotubes that arebranched, crosslinked, and share common walls with one another.Contacting the standing wave at one or more locations with thepreviously deposited layer of the object heats the carbon nanostructuresat the one or more locations and promotes consolidation of the printingcomposition within the object.

In other various embodiments, the present disclosure providesthree-dimensional printing systems which are configured to apply afocused input of microwave radiation during printing. Thethree-dimensional printing systems include a print head, a reservoir ofa printing composition that is fluidly coupled to the print head, astage configured for deposition thereon of the printing composition fromthe print head, and a microwave emitter configured to apply a focusedinput of microwave radiation in proximity to a location where theprinting composition is being deposited from the print head. The stageor the print head is movable and under control of a computer. Theprinting composition contains a solidifiable matrix and a plurality ofcarbon nanostructures dispersed in the solidifiable matrix. The carbonnanostructures include a plurality of carbon nanotubes that arebranched, crosslinked, and share common walls with one another.

The foregoing has outlined rather broadly the features of the presentdisclosure in order that the detailed description that follows can bebetter understood. Additional features and advantages of the disclosurewill be described hereinafter, which form the subject of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionsto be taken in conjunction with the accompanying drawings describingspecific embodiments of the disclosure, wherein:

FIGS. 1A-1C show illustrative depictions of carbon nanotubes that arebranched, crosslinked, and share common walls, respectively;

FIG. 2 shows an illustrative depiction of a carbon nanostructure flakematerial after isolation of carbon nanostructures from a growthsubstrate;

FIG. 3A shows an SEM image of an illustrative carbon nanostructureobtained as a flake material; FIG. 3B shows a high magnification SEMimage of the carbon nanotube morphology in carbon nanostructures;

FIG. 4 shows an illustrative schematic of an object being constructedwith a three-dimensional printing system in which a focused input ofmicrowave radiation is supplied from a microwave emitter locatedtherein;

FIGS. 5-7 show illustrative schematics of an object being constructedwith a three-dimensional printing system in which a focused input ofmicrowave radiation is supplied from a microwave transmission line;

FIG. 8 shows a flow diagram of an illustrative carbon nanostructuregrowth process which employs an exemplary glass or ceramic growthsubstrate;

FIG. 9 shows an illustrative schematic of a transition metalnanoparticle coated with an anti-adhesive layer;

FIG. 10 shows a flow diagram of an illustrative process for isolating acarbon nanostructure from a growth substrate;

FIG. 11 shows an illustrative schematic further elaborating on theprocess demonstrated in FIG. 10;

FIG. 12 shows an illustrative schematic demonstrating how mechanicalshearing can be used to remove a carbon nanostructure and a transitionmetal nanoparticle catalyst from a growth substrate; and

FIG. 13 shows an illustrative schematic demonstrating a carbonnanostructure removal process in which a carbon nanostructure can beisolated from a growth substrate absent a transition metal nanoparticlecatalyst.

DETAILED DESCRIPTION

The present disclosure is directed, in part, to three-dimensionalprinting methods using carbon nanostructures or another microwaveabsorber. The present disclosure is also directed, in part, tothree-dimensional printing systems configured to apply an input ofmicrowave radiation during printing to heat carbon nanostructures oranother microwave absorber.

Carbon nanotubes (CNTs) have been widely proposed for use in a number ofapplications in order to take advantage of their beneficial combinationof chemical, mechanical, electrical, and thermal properties. Variousdifficulties occur when working with individual carbon nanotubes,however. These difficulties include, but are not limited to, poorsolvent solubility, limited dispersibility in composite matrices,inadequate purity, and the like. Without being bound by any theory ormechanism, it is believed that many of these issues can arise due to thestrong van der Waals forces that occur between individual carbonnanotubes, thereby causing them to agglomerate into bundles or ropes, ascommonly known in the art. Although there are various techniquesavailable for de-bundling carbon nanotubes into individual,well-separated members, many of these techniques can detrimentallyimpact the desirable properties of pristine carbon nanotubes. Concernshave also been raised regarding the environmental health and safetyprofile of individual carbon nanotubes due to their small size. Finally,the cost of producing individual carbon nanotubes can be prohibitive forthe commercial viability of these entities in many instances.

One interesting property of carbon nanotubes is their ability tostrongly absorb microwave radiation. Absorption of microwave radiationby the carbon nanotubes causes heating to occur.

In the context of three-dimensional printing, the present inventorsrecognized that if a printed object could be made strongly absorbenttoward microwave radiation, such as by incorporating carbon nanotubes oranother microwave absorber therein, induced heating upon microwaveirradiation during or after printing could be used to affectconsolidation within the printed object and/or to smooth its surface.The terms “part,” “tool” and “article” may be used synonymously hereinwith the term “object.” Although the strong microwave absorption ofcarbon nanotubes would otherwise make them a viable candidate for theforegoing purpose, the general difficulties associated with as-producedcarbon nanotubes and their problematic dispersion characteristics canmake them unsuitable for use in conventional three-dimensional printingmethods.

In order to address the shortcomings presented by ordinary carbonnanotubes, at least some of the present inventors previously developedtechniques to prepare carbon nanostructures in free form following theirinitial growth on a substrate. Illustrative techniques for producingcarbon nanostructures on a growth substrate, followed by releasetherefrom, are described in more detail in commonly owned U.S. PatentApplication Publications 2013/0101495 and 2014/0093728, each of which isincorporated herein by reference in its entirety. As used herein, theterm “carbon nanostructure” will refer to a plurality of carbonnanotubes that exist as a polymeric structure by being interdigitated,branched, crosslinked, and/or sharing common walls with one another.Carbon nanostructures can be considered to have carbon nanotubes presentas a base monomer unit of their polymeric structure. Whereasconventional carbon nanotube growth processes have most often focused onthe production of high-purity carbon nanotubes containing a minimumnumber of defect sites, carbon nanostructure growth processes employnominal carbon nanotube growth rates on the order of several microns persecond to rapidly produce the defect-laden carbon nanostructuremorphology.

Free carbon nanostructures at least partially alleviate certaindifficulties otherwise associated with ordinary carbon nanotubes.Whereas ordinary carbon nanotubes are not easily dispersed in a matrixmaterial due to strong nanotube-to-nanotube interactions, the structuralmorphology of carbon nanostructures allows these entities to be muchmore easily dispersed. Essentially, the as-produced structuralmorphology of carbon nanostructures places the carbon nanotubes thereinin a fixed, pre-exfoliated (i.e., at least partially separated) state,thereby making them much more easily dispersible in a matrix material.Moreover, because carbon nanostructures are macroscopic in size relativeto individual carbon nanotubes and are not prone to shedding ofsubmicron particles, carbon nanostructures can present an improvedenvironmental health and safety profile compared to individual carbonnanotubes. As a further advantage, the rapid growth rates for carbonnanostructures can alleviate the supply issues that can be problematicfor individual carbon nanotubes.

Despite the significant differences existing in their morphology, carbonnanostructures retain many of the advantageous properties thatcharacterize ordinary carbon nanotubes. With respect to thethree-dimensional printing methods disclosed herein, carbonnanostructures retain a strong absorption profile for microwaveradiation, as discussed further herein.

Whereas ordinary carbon nanotubes are considered to be unsuitable foruse in three-dimension printing methods due to their poor dispersioncharacteristics, the present inventors discovered that carbonnanostructures are readily dispersible and can provide satisfactorymatrix heating at readily attainable loading levels that are compatiblewith many aspects of existing three-dimensional printing technology.Further, the present inventors discovered that the strong microwaveabsorption afforded by the carbon nanostructures can help address thedurability and quality issues that presently limit three-dimensionalprinting methods. Specifically, the inventors discovered that byutilizing a printing composition containing carbon nanostructures andapplying microwave radiation during or after fabrication of a printedobject, improved consolidation within the object can be realized throughmicrowave heating. More particularly, by heating the carbonnanostructures, the solidifiable matrix within a previously depositedlayer of the object can become more thoroughly consolidated with asubsequently deposited layer, thereby improving overall structuralintegrity and quality of the printed object. The use of microwaveradiation for this purpose during three-dimensional printing is believedto be entirely unconventional, since the printing compositions typicallyemployed in three-dimensional printing processes do not interactsignificantly with microwaves.

In addition to their ability to promote consolidation of a printedobject through absorption of microwave radiation, carbon nanostructurescan also provide mechanical reinforcement effects as well. Hence, evendiscounting the consolidation effects attainable by practicing theembodiments of the present disclosure, improved structural integrity ofa printed object can be realized.

As a further advantage, the inventors also recognized that microwaveradiation of a sufficiently high frequency can affect highly localized“spot” heating of the solidifiable matrix within a printed object. Bylocally heating the solidifiable matrix in proximity to the locationwhere the printing composition is being deposited, consolidation withinthe object without can be promoted but without inducing excessiveheating throughout the object as a whole. By locally heating the printedobject, rather than the printed object as a whole, heating-induceddeformation effects can be limited. In addition to producing highlylocalized heating effects by modulating the microwave frequency, thedepth of penetration within the printed object can also be adjusted.

As one way to provide a focused input of microwave radiation to aprinted object, the present inventors discovered that a microwavetransmission line can be coupled to the print head of athree-dimensional printer in order to deliver microwave radiation duringprinting. Specifically, the inventors discovered that by establishing astanding wave (a standing microwave) in the transmission line, thestanding wave can attain sufficient amplitude to contact an object atone or more precise locations during or after printing and induceheating of the deposited carbon nanostructures, as discussed above. Bycoupling the microwave transmission line to the print head, the input ofmicrowave radiation to the object can be localized where it is mostneeded for promoting consolidation—in proximity to the print head, wherethe solidifiable matrix can otherwise cool down too rapidly to promoteeffective consolidation. Further aspects of establishing a standing wavein a microwave transmission line and heating an object therewith will bediscussed in greater detail below.

In various embodiments, the methods and systems of the presentdisclosure utilize a printing composition containing a solidifiablematrix and a plurality of carbon nanostructures. The carbonnanostructures include a plurality of carbon nanotubes in which thecarbon nanotubes are branched, crosslinked, and share common walls withone another. It is to be recognized that every carbon nanotube in theplurality of carbon nanotubes does not necessarily have the foregoingstructural features of branching, crosslinking, and sharing commonwalls. Rather, the plurality of carbon nanotubes as a whole collectivelypossesses these structural features. That is, within carbonnanostructures, at least a portion of the carbon nanotubes are branched,at least a portion of the carbon nanotubes are crosslinked, and at leasta portion of the carbon nanotubes share common walls. FIGS. 1A-1C showillustrative depictions of carbon nanotubes 1-3 that are branched,crosslinked, and share common walls, respectively. Shared-wall carbonnanotubes 3 are not merely carbon nanotubes that are abuttedside-by-side with one another. Rather, in shared-wall carbon nanotubes3, at least the outer carbon nanotube layer of a first carbon nanotubeis indistinguishable from and contiguous with that of a second carbonnanotube. The carbon nanotubes within the carbon nanostructures can beformed with branching, crosslinking, and sharing common walls with oneanother during formation of the carbon nanostructures on a growthsubstrate, as discussed hereinafter.

Carbon nanostructures can have a web-like morphology that results in alow, but readily modified, bulk density. As-produced carbonnanostructures can have an initial bulk density ranging between about0.003 g/cm³ to about 0.015 g/cm³. Densification and/or coating toproduce a carbon nanostructure flake material or a like morphology canraise the bulk density to a range between about 0.1 g/cm³ to about 0.15g/cm³. Further compaction can raise the bulk density to an upper limitof about 1 g/cm³, with chemical modifications to the carbonnanostructures raising the bulk density to an upper limit of about 1.2g/cm³. In any event, the carbon nanostructures can remain readilydispersible in a solidifiable matrix for use in printing compositions ofthe present disclosure.

At least a portion of the carbon nanotubes can be aligned substantiallyparallel to one another in the carbon nanostructures. Without beingbound by any theory or mechanism, it is believed that the formation ofcarbon nanotubes on a growth substrate under carbon nanostructure growthconditions can result in substantially vertical growth of at least amajority of the carbon nanotubes from the substrate surface. Thestructural features of branching, crosslinking, and shared carbonnanotube walls can become more prevalent as the growth densityincreases, particularly at locations on the carbon nanotubes that arefurther removed from the growth substrate. After removal of the carbonnanostructures from the growth substrate, the substantially parallelalignment of the carbon nanotubes can be maintained, as discussed below.

In some embodiments, the carbon nanostructures can be free of a growthsubstrate and in the form of a flake material after removal from theinitial growth substrate. As used herein, the term “flake material”refers to a discrete particle having finite dimensions. FIG. 2 shows anillustrative depiction of a carbon nanostructure flake material afterisolation of carbon nanostructures from a growth substrate. Flakestructure 10 can have first dimension 11 that is in a range from about 1nm to about 35 μm thick, particularly about 1 nm to about 500 nm thick,including any value in between and any fraction thereof. Flake structure10 can have second dimension 12 that is in a range from about 1 micronto about 750 microns tall, including any value in between and anyfraction thereof. Flake structure 10 can have third dimension 13 that isonly limited in size based on the length of the growth substrate uponwhich the carbon nanostructures are initially formed. For example, insome embodiments, the process for growing carbon nanostructures on agrowth substrate can take place on a tow or roving of a fiber-basedmaterial of spoolable dimensions. The carbon nanostructure growthprocess can be continuous, and the carbon nanostructures can extend theentire length of a spool of fiber. Thus, in some embodiments, thirddimension 13 can be in a range from about 1 m to about 10,000 m wide.Again, third dimension 13 can be very long because it represents thedimension that runs along the axis of the growth substrate upon whichthe carbon nanostructures are formed. Third dimension 13 can also bedecreased to any desired length less than 1 m. For example, in someembodiments, third dimension 13 can be on the order of about 1 micron toabout 10 microns, or about 10 microns to about 100 microns, or about 100microns to about 500 microns, or about 500 microns to about 1 cm, orabout 1 cm to about 100 cm, or about 100 cm to about 500 cm, up to anydesired length, including any amount between the recited ranges and anyfractions thereof. Since the growth substrates upon which carbonnanostructures are formed can be quite large, exceptionally highmolecular weight carbon nanostructures can be produced.

Referring still to FIG. 2, flake structure 10 can include a webbednetwork of carbon nanotubes 14 in the form of a carbon nanotube polymer(i.e., a “carbon nanopolymer”) having a molecular weight in a range fromabout 15,000 g/mol to about 150,000 g/mol, including all values inbetween and any fraction thereof. In some embodiments, the upper end ofthe molecular weight range can be even higher, including about 200,000g/mol, about 500,000 g/mol, or about 1,000,000 g/mol. The highermolecular weights can be associated with carbon nanostructures that aredimensionally long. In various embodiments, the molecular weight canalso be a function of the predominant carbon nanotube diameter andnumber of carbon nanotube walls present within the carbon nanostructure.In some embodiments, the carbon nanostructures can have a crosslinkingdensity ranging between about 2 mol/cm³ to about 80 mol/cm³. Thecrosslinking density can be a function of the carbon nanostructuregrowth density on the surface of the growth substrate as well as thecarbon nanostructure growth conditions.

FIG. 3A shows an SEM image of an illustrative carbon nanostructureobtained as a flake material. The carbon nanostructure shown in FIG. 3Aexists as a three dimensional microstructure due to the entanglement andcrosslinking of its aligned carbon nanotubes. FIG. 3B shows a highmagnification SEM image of the carbon nanotube morphology in carbonnanostructures. Again, the aligned morphology is reflective of theformation of the carbon nanotubes on a growth substrate under rapidcarbon nanotube growth conditions (e.g., several microns per second,such as about 2 microns per second to about 10 microns per second),thereby inducing substantially perpendicular carbon nanotube growth fromthe growth substrate and the accompanying carbon nanostructuremorphology. Additional details regarding carbon nanostructures andmethods for their production are discussed hereinbelow.

Although carbon nanostructures can be effectively utilized as themicrowave absorber in the printing compositions of the presentdisclosure, it is to be recognized that other types of dispersiblemicrowave absorbers can be utilized as well. For example, in someembodiments, the microwave absorption properties of metals can beutilized in order to realize the features and advantages of the presentdisclosure. In particular embodiments, a metallic microwave absorbersuitable for inclusion in printing compositions of the presentdisclosure can be metal nanoparticles. As used herein, the term“nanoparticle” will refer to a particulate material having an equivalentspherical diameter of about 100 nm or less, although nanoparticles neednot necessarily be spherical in shape. Suitable techniques forproduction and isolation of various types of metal nanoparticles will befamiliar to one having ordinary skill in the art. Metal nanoparticlescan be readily dispersible within the solidifiable matrix of printingcompositions, much like carbon nanostructures. Although certainembodiments herein are described in reference to microwave heating ofcarbon nanostructures, it is to be recognized that metal nanoparticlescan be substituted for carbon nanostructures or used in combination withcarbon nanostructures in any particular configuration herein.

The solidifiable matrix of the printing composition can include anymaterial that can be conventionally deposited in a three-dimensionalprinting process and in which the carbon nanostructures can beeffectively dispersed. In some embodiments, the solidifiable matrix canbe a thermoplastic polymer. In more particular embodiments, suitablethermoplastic polymers that can be used in three-dimensional printinginclude, for example, polyketones, acrylonitrile-butadiene-styrene(ABS), polyetheretherketones (PEEK), polyamides, polyolefins (e.g.,polyethylene, polypropylene and the like), polyethyleneimine (PEI),polycarbonates, and combinations thereof. In other various embodiments,the solidifiable material can be a curable material such as a polymerresin, a ceramic precursor, cement, and the like. Curable materials canbe deposited in a partially solidified form and under further curingupon undergoing microwave-induced heating.

In various embodiments, a loading of carbon nanostructures in thesolidifiable matrix can be less than about 30% by weight. In moreparticular embodiments, the carbon nanostructure loading can rangebetween about 0.1% to about 30% by weight, or between about 1% to about25% by weight, or between about 5% to about 20% by weight, or betweenabout 1% to about 15% by weight, or between about 5% to about 10% byweight.

In various embodiments, methods described herein can include depositinga printing composition in a layer-by-layer deposition process, and whiledepositing the printing composition, applying a focused input ofmicrowave radiation in proximity to a location where the printingcomposition is being deposited. The printing composition includes asolidifiable matrix and a plurality of carbon nanostructures dispersedin the solidifiable matrix. The carbon nanostructures include aplurality of carbon nanotubes that are branched, crosslinked and sharecommon walls with one another, as described above. By applying thefocused input of microwave radiation, the carbon nanostructures areheated at the location and consolidation of the printing composition ispromoted within an object being produced by the layer-by-layerdeposition process.

As used herein, the term “microwave radiation” refers to the region ofthe electromagnetic spectrum having a frequency residing with a range ofabout 300 GHz to about 300 MHz (wavelengths of 1 mm to 1 m,respectively). In more particular embodiments of the present disclosure,the microwave radiation can have a frequency of about 1 GHz or higher,specifically within the range of about 10 GHz to about 300 GHz(wavelengths of 10 mm to 1 mm, respectively) or within the range ofabout 10 GHz to about 40 GHz (wavelengths of 10 mm to 7.5 mm,respectively). Microwave radiation having a frequency above about 10 GHzcan be very effectively focused due to its small wavelength, therebyallowing extremely localized heating to take place within a printedobject. In addition, microwave radiation having a frequency above about10 GHz can exhibit a very low extent of penetration into the interior ofan object, thereby allowing the carbon nanostructures in proximity tothe exterior of a printed object to undergo selective microwave heating.That is, in some embodiments, the focused input of microwave radiationcan have a frequency such that interior portions of the printed objectremain substantially unheated through direct absorption of microwaves,although some heating may occur through conduction.

At lower microwave frequencies, a greater extent of penetration andinterior heating within the printed object can be realized. For example,at lower microwave frequencies within the range of about 1 GHz to about10 GHz, the carbon nanostructures within the interior of the object canundergo heating in preference to those in proximity to the object'sexterior. Moreover, because of the larger wavelengths at lower microwavefrequencies, the heating effect is less localized.

The object being produced by the layer-by-layer deposition process isnot considered to be particularly limited in identity, structure orsize. Any structure that can be modeled as a three-dimensional CADdrawing can allow computer control to be realized for production of aprinted object. Printed objects of arbitrary size can be prepared byusing a three-dimensional printer having a suitably large stage fordeposition of the printing composition.

As used herein, the term “in proximity to” refers to a location of aprinted object to which microwave radiation is applied such that thecarbon nanostructures at the location undergo heating and remain aboveambient temperature when the printing composition is deposited thereon.

In various embodiments, the location to which the microwave radiation isapplied is a previously deposited layer of the printed object. Heatingof the carbon nanostructures in the previously deposited layer with themicrowave radiation can promote consolidation of the solidifiable matrixtherein with the solidifiable matrix in a subsequently deposited layerof the printing composition. Specifically, heating of the solidifiablematrix in the previously deposited layer can soften or liquefy thesolidifiable matrix such that it undergoes consolidation more readilywith the solidifiable matrix undergoing deposition.

In some embodiments, the focused input of microwave radiation can beapplied at the location where the printing composition is beingdeposited. For example, when depositing the printing composition upon apreviously deposited layer of the object, the focused input of microwaveradiation can be deposited directly below a print head that isdepositing the printing composition.

In other various embodiments, the focused input of microwave radiationcan be applied at a location that is nearby a location where theprinting composition is currently being deposited. By heating the carbonnanostructures at a nearby location, the solidifiable matrix can stillremain warm enough to promote consolidation by the time a print headdepositing the printing composition reaches the location where themicrowave radiation was previously applied.

In more particular embodiments of the present disclosure, the printingcomposition can be deposited as a plurality of droplets (e.g., sprayed),with at least a portion of the droplets becoming consolidated with apreviously deposited layer of the object. That is, droplets in apreviously deposited layer of the object can become consolidated withthe droplets in a subsequently deposited layer, thereby bonding the twolayers together. In addition, by applying a focused input of microwaveradiation directly in a location where the printing composition is beingdeposited, more effective droplet consolidation within thejust-deposited layer of the object can also be realized.

Suitable microwave emitters for providing a focused input of microwaveradiation can include both low-power and high-power microwave emitters.High-power microwave emitters that can be suitable for this purposeinclude, for example, magnetrons, klystrons, traveling-wave tubes, andgyrotrons. Low-power microwave emitters that can be suitable include,for example, field-effect transistors, tunnel diodes, Gunn diodes,impact ionization avalanche transit-time diodes, masers and the like. Afeed horn or emitter horn sized for the particular wavelength ofmicrowave radiation may be used. Microwave transmission lines and likestructures, including microwave waveguides, can represent a particularlysuitable type of microwave emitter for producing a focused input ofmicrowave radiation in some embodiments of the present disclosure.

FIG. 4 shows an illustrative schematic of an object being constructedwith a three-dimensional printing system in which a focused input ofmicrowave radiation is supplied from a microwave emitter locatedtherein. As shown in FIG. 4, object 30 is deposited layer-by-layer uponstage 32. Object 30 contains lower layer 34 and upper layer 36. Asdepicted in FIG. 4, upper layer 36 is incomplete and is in the processof being deposited from print head 40. Print head 40 receives a supplyof a printing composition from reservoir 42 via line 43 and depositsstream 44 of the printing composition to continue buildup of upper layer36. Microwave emitter 46 supplies microwave radiation 47 in proximity tothe location where stream 44 is being deposited. Input of microwaveradiation 47 from microwave emitter 46 results in localized heating oflower layer 34 as the printing composition is deposited thereon, therebypromoting consolidation between lower layer 34 and upper layer 36 into asubstantially homogenous structure. Although FIG. 4 has shown microwaveradiation 47 being focused directly below print head 40, it can also befocused in a nearby location, as discussed in more detail above.

Print head 40 and microwave emitter 46 can move two-dimensionally orthree-dimensionally in concert with one another to maintain the focusedinput of microwave radiation at a location proximate to the locationwhere the printing composition is being deposited. Movement of printhead 40 or microwave emitter 46 can take place under computer control inresponse to a CAD drawing used for construction of object 30. Thevertical separation distance between print head 40 and object 30 can beadjusted to accommodate the degree of spread that occurs in stream 44,thereby influencing the precision with which object 30 can befabricated. The frequency of the microwave radiation and the distancebetween microwave emitter 46 and object 30 can also be varied tomodulate the printing and consolidation process.

In more particular embodiments of the present disclosure, the presentinventors discovered that a focused input of microwave radiation can besupplied by a microwave transmission line located upon the same side ofthe object from which the printing composition is being deposited. Moreparticularly, as discussed in greater detail hereinafter, a standingwave can be established in the microwave transmission line such that thestanding wave's amplitude is sufficiently great to contact the object atone or more locations and heat the carbon nanostructures therein. Bymodulating the frequency of the microwave radiation input to themicrowave transmission line, as well as the length of the microwavetransmission line itself, the locations at which the standing wavecontacts the object can be very precisely modulated. The microwavefrequency can be similar to those discussed above, such as within arange of about 100 MHz to about 50 GHz, although higher or lowerfrequencies may also be suitable. The relative proximity of themicrowave transmission line and the printed object to one another canalso be adjusted to alter the extent of contact between the printedobject and the standing wave.

FIGS. 5-7 show illustrative schematics of an object being constructedwith a three-dimensional printing system in which a focused input ofmicrowave radiation is supplied from a microwave transmission line.FIGS. 5-7 share several elements in common with FIG. 4 and may be betterunderstood by reference thereto. As shown in FIG. 5, object 30 isdeposited layer-by-layer upon stage 32. Object 30 contains lower layer34 and upper layer 36. As depicted in FIG. 5, upper layer 36 isincomplete and is in the process of being deposited from print head 40.Print head 40 is coupled to microwave transmission line 50, receives asupply of a printing composition from reservoir 42 via line 43 anddeposits stream 44 of the printing composition to continue buildup ofupper layer 36.

Transmission line 50 extends between microwave input 52 and reflectiveload 54. Transmission line 50 is in electrical communication with object30 by electrical connectors 39 a and 39 b, as depicted in FIGS. 5-7. Inalternative embodiments, an electrical connection can similarly beestablished with stage 32, provided that stage 32 is electricallyconductive. Electrical connectors 39 a and 39 b can be any structurethat establishes a movable electrical connection with object 30 or stage32. For example, electrical connectors 39 a and 39 b can be aspring-loaded conductive plate or leaf (39 a) or a conductive roller (39b).

Upon interacting microwave radiation with reflective load 54 located atthe terminus of transmission line 50, backward microwave reflectionestablishes standing wave 56 in transmission line 50. Standing wave 56is generally sinusoidal in shape and its amplitude is sufficiently greatthat it is no longer constrained within transmission line 50. By placingtransmission line 50 sufficiently close to object 30 and/or increasingthe amplitude of standing wave 56, standing wave 56 can contact object30 at one or more locations. At the locations where standing wave 56contacts object 30, localized heating occurs due to interaction ofmicrowave radiation with the carbon nanostructures. Although themicrowave radiation may have a limited depth of penetration into object30, deeper penetration of heat can occur via conduction.

Although FIG. 5 has depicted standing wave 56 as establishing multiplepoints of contact with object 30 and inducing heating therein, this isnot necessary the case. As discussed above, standing wave 56 can bemodulated by modifying the frequency of the microwave radiation input totransmission line 50 and/or the distance between microwave input 52 andreflective load 54. Modulation of the standing wave in this manner canmodify the number and location of the points of contact by changing thenumber of nodes in the standing wave. For example, in some embodiments,the standing wave can contact the object at substantially a singlelocation, as depicted in FIG. 6 and discussed in more detail below.Moreover, modulation of the amplitude of standing wave 56 can determinewhether the standing wave contacts lower layer 34 in addition to upperlayer 36. As depicted in FIG. 5, standing wave 56 contacts upper layer36 (a previously deposited layer of the object), possibly in asubstantially tangential manner. By increasing the depth of penetrationinto object 30 (e.g., by moving transmission line 50 closer to object 30and/or by increasing the amplitude of standing wave 30), a larger areaof object 30 can undergo induced heating.

As shown in FIG. 6, standing wave 56 can be modulated to contact theobject at a single location. In the particular configuration shown inFIG. 6, modulation of standing wave 56 includes shortening transmissionline 50 by moving microwave input 52 and reflective load 54 closer toone another. However, modulation can also take place solely bymodulation of the input frequency of the microwave radiation, asdiscussed above. In the particular embodiment shown in FIG. 6, standingwave 56 contacts lower layer 34 (a previously deposited layer of object30) at a location where the printing composition is being deposited. Byheating the previously deposited layer of object 30 where the printingcomposition is being presently deposited, better consolidation can berealized, as discussed in further detail above. Similarly, a standingwave that contacts object 30 at multiple locations, such as depicted inFIG. 5, can heat a previously deposited layer of the object at locationwhere the printing composition is being deposited. In this case,however, multiple points of interior heating within object 30 would bepresent (i.e., at the interface between layer 34 and 36).

FIG. 7 shows still another alternative configuration of the presentdisclosure. Specifically, as shown in FIG. 7, standing wave 56 cancontact lower layer 34 in a location just before printing compositiondeposits upper layer 36 thereon. By heating lower layer 34 just beforedepositing upper layer 34 thereon, lower layer 34 can still be heated toa sufficient extent to realize the benefits of the disclosure providedherein.

As shown in FIGS. 5-7, print head 40 is coupled to microwavetransmission line 50. In some embodiments, print head 40 can move alongthe length of microwave transmission line 50 in the course of depositingobject 30. In other embodiments, print head 40, transmission line 50,microwave input 52 and reflective load 54 can be moved as a unit in thecourse of depositing object 30. In still other embodiments, print head40, transmission line 50, microwave input 52 and reflective load 54 canremain static in the course of depositing object 30, and stage 32 caninstead be moved in order to deposit the printing composition in adesired location. In any event, the movable entities can be controlledby a computer based upon a CAD drawing or pattern for the object beingprinted. Suitable mechanical means for affecting movement and means forcomputer control will be evident to one having ordinary skill in theart.

Accordingly, in more specific embodiments of the present disclosure,three-dimensional printing methods can include: depositing a printingcomposition in a layer-by-layer deposition process in which the printingcomposition is deposited from a print head that is coupled to amicrowave transmission line, establishing a standing wave in themicrowave transmission line, and while depositing the printingcomposition, contacting the standing wave at one or more locations witha previously deposited layer of an object being produced by thelayer-by-layer deposition process. The printing composition contains asolidifiable matrix and a plurality of carbon nanostructures dispersedin the solidifiable matrix. The carbon nanostructures include aplurality of carbon nanotubes that are branched, crosslinked, and sharecommon walls with one another. Contacting the standing wave at one ormore locations with the previously deposited layer of the object heatsthe carbon nanostructures at the one or more locations and promotesconsolidation of the printing composition within the object.

In order to provide a more thorough understanding of the presentdisclosure, exemplary processes for producing carbon nanostructures on agrowth substrate and releasing the carbon nanostructures in free formare described in greater detail hereinbelow.

In some embodiments, processes described herein can include preparing acarbon nanostructure on a growth substrate with one or more provisionsfor removal of the carbon nanostructure once synthesis of the carbonnanostructure is complete. The provision(s) for removing the carbonnanostructure from the growth substrate can include one or moretechniques selected from the group consisting of: (i) providing ananti-adhesive coating on the growth substrate, (ii) providing ananti-adhesive coating on a transition metal nanoparticle catalystemployed in synthesizing the carbon nanostructure, (iii) providing atransition metal nanoparticle catalyst with a counter ion that etchesthe growth substrate, thereby weakening the adherence of the carbonnanostructure to the growth substrate, and (iv) conducting an etchingoperation after carbon nanostructure synthesis is complete to weakenadherence of the carbon nanostructure to the growth substrate.Combinations of these techniques can also be used. In combination withthese techniques, various fluid shearing or mechanical shearingoperations can be carried out to affect the removal of the carbonnanostructure from the growth substrate.

In some embodiments, processes disclosed herein can include removing acarbon nanostructure from a growth substrate. In some embodiments,removing a carbon nanostructure from a growth substrate can includeusing a high pressure liquid or gas to separate the carbon nanostructurefrom the growth substrate, separating contaminants derived from thegrowth substrate (e.g., fragmented growth substrate) from the carbonnanostructure, collecting the carbon nanostructure with air or from aliquid medium with the aid of a filter medium, and isolating the carbonnanostructure from the filter medium. In various embodiments, separatingcontaminants derived from the growth substrate from the carbonnanostructure can take place by a technique selected from the groupconsisting of cyclone filtering, density separation, size-basedseparation, and any combination thereof. The foregoing processes aredescribed in more detail hereinbelow.

FIG. 8 shows a flow diagram of an illustrative carbon nanostructuregrowth process 400, which employs an exemplary glass or ceramic growthsubstrate 410. It is to be understood that the choice of a glass orceramic growth substrate is merely exemplary, and the substrate can alsobe metal, an organic polymer (e.g., aramid), basalt fiber, or carbon,for example. In some embodiments, the growth substrate can be a fibermaterial of spoolable dimensions, thereby allowing formation of thecarbon nanostructure to take place continuously on the growth substrateas the growth substrate is conveyed from a first location to a secondlocation. Carbon nanostructure growth process 400 can employ growthsubstrates in a variety of forms such as fibers, tows, yarns, woven andnon-woven fabrics, sheets, tapes, belts and the like. For convenience incontinuous syntheses, tows and yarns are particularly convenient fibermaterials.

Referring still to FIG. 8, such a fiber material can be meted out from apayout creel at operation 420 and delivered to an optional desizingstation at operation 430. Desizing is ordinarily conducted whenpreparing carbon nanostructure-infused fiber materials in order toincrease the degree of infusion of the carbon nanostructure to the fibermaterial. However, when preparing an isolated carbon nanostructure,desizing operation 430 can be skipped, for example, if the sizingpromotes a decreased degree of adhesion of the transition metalnanoparticle catalyst and/or carbon nanostructure to the growthsubstrate, thereby facilitating removal of the carbon nanostructure.Numerous sizing compositions associated with fiber substrates cancontain binders and coupling agents that primarily provide anti-abrasiveeffects, but typically do not exhibit exceptional adhesion to fibersurface. Thus, forming a carbon nanostructure on a growth substrate inthe presence of a sizing can actually promote subsequent isolation ofthe carbon nanostructure in some embodiments. For this reason, it can bebeneficial to skip desizing operation 430, in some embodiments.

In some embodiments, an additional coating application can take place atoperation 440. Additional coatings that can be applied in operation 440include, for example, colloidal ceramics, glass, silanes, or siloxanesthat can decrease catalyst and/or carbon nanostructure adhesion to thegrowth substrate. In some embodiments, the combination of a sizing andthe additional coating can provide an anti-adhesive coating that canpromote removal of the carbon nanostructure from the growth substrate.In some embodiments, the sizing alone can provide sufficientanti-adhesive properties to facilitate carbon nanostructure removal fromthe growth substrate, as discussed above. In some embodiments, theadditional coating provided in operation 440 alone can providesufficient anti-adhesive properties to facilitate carbon nanostructureremoval from the growth substrate. In still further embodiments, neitherthe sizing nor the additional coating, either alone or in combination,provides sufficient anti-adhesive properties to facilitate carbonnanostructure removal. In such embodiments, decreased adhesion of thecarbon nanostructure to the growth substrate can be attained byjudicious choice of the transition metal nanoparticles used to promotegrowth of the carbon nanostructure on the growth substrate.Specifically, in some such embodiments, operation 450 can employ acatalyst that is specifically chosen for its poor adhesivecharacteristics.

Referring still to FIG. 8, after optional desizing operation 430 andoptional coating operation 440, catalyst is applied to the growthsubstrate in operation 450, and carbon nanostructure growth is affectedthrough a small cavity CVD process in operation 460. The resultingcarbon nanostructure-infused growth substrate (i.e., a carbonnanostructure-infused fiber material) can be wound for storage andsubsequent carbon nanostructure removal or immediately taken into acarbon nanostructure isolation process employing a harvester, asindicated in operation 470.

In some embodiments, the growth substrate can be modified to promoteremoval of a carbon nanostructure therefrom. In some embodiments, thegrowth substrate used for producing a carbon nanostructure can bemodified to include an anti-adhesive coating that limits adherence ofthe carbon nanostructure to the growth substrate. The anti-adhesivecoating can include a sizing that is commercially applied to the growthsubstrate, or the anti-adhesive coating can be applied after receipt ofthe growth substrate. In some embodiments, a sizing can be removed fromthe growth substrate prior to applying an anti-adhesive coating. Inother embodiments, a sizing can be applied to a growth substrate inwhich a sizing is present.

In some embodiments, the carbon nanostructure can be grown on the growthsubstrate from a catalyst that includes a plurality of transition metalnanoparticles, as generally described hereinbelow. In some embodiments,one mode for catalyst application onto the growth substrate can bethrough particle adsorption, such as through direct catalyst applicationusing a liquid or colloidal precursor-based deposition. Suitabletransition metal nanoparticle catalysts can include any d-blocktransition metal or d-block transition metal salt. In some embodiments,a transition metal salt can be applied to the growth substrate withoutthermal treatments. In other embodiments, a transition metal salt can beconverted into a zero-valent transition metal on the growth substratethrough a thermal treatment.

In some embodiments, the transition metal nanoparticles can be coatedwith an anti-adhesive coating that limits their adherence to the growthsubstrate. As discussed above, coating the transition metalnanoparticles with an anti-adhesive coating can also promote removal ofthe carbon nanostructure from the growth substrate following synthesisof the carbon nanostructure. Anti-adhesive coatings suitable for use inconjunction with coating the transition metal nanoparticles can includethe same anti-adhesive coatings used for coating the growth substrate.FIG. 9 shows an illustrative schematic of a transition metalnanoparticle coated with an anti-adhesive layer. As shown in FIG. 9,coated catalyst 500 can include core catalyst particle 510 overcoatedwith anti-adhesive layer 520. In some embodiments, colloidalnanoparticle solutions can be used in which an exterior layer about thenanoparticle promotes growth substrate to nanoparticle adhesion butdiscourages carbon nanostructure to nanoparticle adhesion, therebylimiting adherence of the carbon nanostructure to the growth substrate.

FIG. 10 shows a flow diagram of an illustrative process for isolating acarbon nanostructure from a growth substrate. As shown in FIG. 10,process 600 begins with a carbon nanostructure-infused fiber beingprovided in operation 610. Non-fibrous growth substrates onto which acarbon nanostructure has been grown can be used in a like manner. Fluidshearing can be conducted at operation 620 using a gas or a liquid inorder to accomplish removal of the carbon nanostructure from the fibermaterial. In some cases, fluid shearing can result in at least a portionof the fiber material being liberated from the bulk fiber andincorporated with the free carbon nanostructure, while not being adheredthereto. If needed, in operation 630, the liberated carbon nanostructurecan be subjected to cyclonic/media filtration in order to remove thenon-adhered fiber material fragments. Density-based or size-basedseparation techniques can also be used to bring about separation of thecarbon nanostructure from the non-adhered fiber material. In the case ofgas shearing, the carbon nanostructure can be collected in dry form on afilter medium in operation 645. The resultant dry flake materialcollected in operation 645 can be subjected to any optional furtherchemical or thermal purification, as outlined further in FIG. 10. In thecase of liquid shearing, the liquid can be collected in operation 640,and separation of the carbon nanostructure from the liquid can takeplace in operation 650, ultimately producing a dry flake material inoperation 660. The carbon nanostructure flake material isolated inoperation 660 can be similar to that produced in operation 645. Afterisolating the carbon nanostructure flake material in operation 660, itcan be ready for packaging and/or storage in operation 695. In processesemploying gas shearing to remove the carbon nanostructure, the carbonnanostructure can be dry collected in a filter at operation 645. Priorto packaging and/or storage in operation 695, the crude product formedby either shearing technique can undergo optional chemical and/orthermal purification in operation 670. These purification processes canbe similar to those conducted when purifying traditional carbonnanotubes. By way of example, purification conducted in operation 670can involve removal of a catalyst used to affect carbon nanostructuregrowth, such as, for example, through treatment with liquid bromine.Other purification techniques can be envisioned by one having ordinaryskill in the art.

Referring still to FIG. 10, the carbon nanostructure produced by eithershearing technique can undergo further processing by cutting or fluffingin operation 680. Such cutting and fluffing can involve mechanical ballmilling, grinding, blending, chemical processes, or any combinationthereof. Further optionally, in operation 690, the carbon nanostructurecan be further functionalized using any technique in which carbonnanotubes are normally modified or functionalized. Suitablefunctionalization techniques in operation 690 can include, for example,plasma processing, chemical etching, and the like. Functionalization ofthe carbon nanostructure in this manner can produce chemical functionalgroup handles that can be used for further modifications. For example,in some embodiments, a chemical etch can be employed to form carboxylicacid groups on the carbon nanostructure that can be used to bring aboutcovalent attachment to any number of further entities including, forexample, the matrix material of a composite material. In this regard, afunctionalized carbon nanostructure can provide a superior reinforcementmaterial in a composite matrix, since it can provide multiple sites forcovalent attachment to the composite's matrix material in alldimensions.

In addition to facilitating the covalent attachment of a carbonnanostructure to the matrix of a composite material, functionalizationof a carbon nanostructure can also allow other groups to be covalentlyattached to the carbon nanostructure. In some embodiments, access toother covalently linked entities such as synthetic or biopolymers can berealized via functional group handles produced in post-processing carbonnanostructure functionalization. For example, a carbon nanostructure canbe linked to polyethylene glycol (e.g., through ester bonds formed fromcarboxylic acid groups on the carbon nanostructure) to provide aPEGylated carbon nanostructure, which can confer improved watersolubility to the carbon nanostructure. In some embodiments, the carbonnanostructure can provide a platform for covalent attachment tobiomolecules to facilitate biosensor manufacture. In this regard, thecarbon nanostructure can provide improved electrical percolationpathways for enhanced detection sensitivity relative to other carbonnanotube-based biosensors employing individualized carbon nanotubes oreven conventional carbon nanotube forests. Biomolecules of interest forsensor development can include, for example, peptides, proteins,enzymes, carbohydrates, glycoproteins, DNA, RNA, and the like.

FIG. 11 shows an illustrative schematic further elaborating on theprocess demonstrated in FIG. 10. As illustrated in process 700 of FIG.11, a single spool or multiple spools of a carbon nanostructure-ladenfiber-type substrate is fed in operation 710 to removal chamber 712using a pay-out and take-up system. Removal of the carbon nanostructurefrom the fiber-type substrate can be affected with a single or severalpressurized air source tools 714, such as an air knife or air nozzle atoperation 720. Such air source tools can be placed generallyperpendicular to the spool(s), and the air can then be directed on tothe fiber-type substrate carrying the carbon nanostructure. In someembodiments, the air source tool can be stationary, while in otherembodiments, the air source tool can be movable. In embodiments wherethe air source tool is movable, it can be configured to oscillate withrespect to the surface of the fiber-type substrate to improve theremoval efficiency. Upon air impact, fiber tows and other bundledfiber-type substrates can be spread, thereby exposing additional surfacearea on the substrate and improving removal of the carbon nanostructure,while advantageously avoiding mechanical contact. In some embodiments,the integrity of the substrate can be sufficient to recycle thesubstrate in a continuous cycle of carbon nanostructure synthesis andremoval. Thus, in some embodiments, the substrate can be in the form ofa belt or a loop in which a carbon nanostructure is synthesized on thesubstrate, subsequently removed downstream, and then recycled foradditional growth of a new carbon nanostructure in the location wherethe original carbon nanostructure was removed. In some embodiments,removal of the original carbon nanostructure can result in removal ofthe surface treatment that facilitated carbon nanostructure removal.Thus, in some embodiments, the substrate can again be modified afterremoval of the original carbon nanostructure to promote removal of thenew carbon nanostructure, as generally performed according to thesurface modification techniques described herein. The surface treatmentperformed on the substrate after the original carbon nanostructure isremoved can be the same or different as the original surface treatment.

In some embodiments, the integrity of the substrate can be compromisedduring carbon nanostructure removal, and at least a portion of thesubstrate can become admixed with the carbon nanostructure while nolonger being adhered thereto. Referring still to FIG. 11, fragmentedsubstrate that has become admixed with the isolated carbon nanostructurecan be removed in operation 730. In FIG. 11, operation 730 is depictedas taking place by cyclonic filtration, but any suitable solidsseparation technique can be used. For example, in some embodiments,sieving, differential settling, or other size-based separations can beperformed. In other embodiments, density-based separations can beperformed. In still other embodiments, a chemical reaction may be used,at least in part, to affect separation of the carbon nanostructure fromgrowth substrate that is not adhered to the carbon nanostructure.Although FIG. 11 has depicted a single cyclonic filtration, multiplevacuum and cyclonic filtration techniques can be used in series,parallel, or any combination thereof to remove residual fragmentedgrowth substrate from the carbon nanostructure. Such techniques canemploy multiple stages of filter media and/or filtration rates toselectively capture the fragmented growth substrate while allowing thecarbon nanostructure to pass to a collection vessel. The resultantcarbon nanostructure can be either collected dry at operation 740 orcollected as a wet sludge at operation 750. In some embodiments, thecarbon nanostructure can be processed directly following the removal offragmented growth substrate in operation 730 and packed into a storagevessel or shippable container in packaging operation 760. Otherwise,packaging can follow dry collection operation 740 or wet collectionoperation 750.

In embodiments where wet processing is employed, the carbonnanostructure can be mixed with about 1% to about 40% solvent in waterand passed through a filter or like separation mechanism to separate thecarbon nanostructure from the solvent. The resultant separated carbonnanostructure can be dried and packed or stored “wet” as a dispersion ina fluid phase. It has been observed that unlike individualized carbonnanotube solutions or dispersions, carbon nanostructures canadvantageously form stable dispersions. In some embodiments, stabledispersions can be achieved in the absence of stabilizing surfactants,even with water as solvent. In some or other embodiments, a solvent canbe used in combination with water during wet processing. Suitablesolvents for use in conjunction with wet processing can include, but arenot limited to, isopropanol (IPA), ethanol, methanol, and water.

As an alternative to fluid shearing, mechanical shearing can be used toremove the carbon nanostructure from the growth substrate in someembodiments. FIG. 12 shows an illustrative schematic demonstrating howmechanical shearing can be used to remove a carbon nanostructure and atransition metal nanoparticle catalyst from a growth substrate. As shownin FIG. 12, carbon nanostructure removal process 800 can employmechanical shearing force 810 to remove both the carbon nanostructureand the transition metal nanoparticle catalyst from growth substrate 830as monolithic entity 820. In some such embodiments, sizing and/oradditional anti-adhesive coatings can be employed to limit carbonnanostructure and/or nanoparticle adhesion to the growth substrate,thereby allowing mechanical shear or another type of shearing force tofacilitate removal of the carbon nanostructure from the growthsubstrate. In some embodiments, mechanical shear can be provided bygrinding the carbon nanostructure-infused fiber with dry ice.

As another alternative to fluid shearing, in some embodiments,sonication can be used to remove the carbon nanostructure from thegrowth substrate.

In some embodiments, the carbon nanostructure can be removed from thegrowth substrate without substantially removing the transition metalnanoparticle catalyst. FIG. 13 shows an illustrative schematicdemonstrating carbon nanostructure removal process 900 in which a carbonnanostructure can be isolated from a growth substrate absent atransition metal nanoparticle catalyst. As shown in FIG. 13, carbonnanostructure 940 can be grown on growth substrate 920 using implantedtransition metal nanoparticle catalyst 910. Thereafter, shear removal930 of carbon nanostructure 940 leaves transition metal nanoparticlecatalyst 910 behind on growth substrate 920. In some such embodiments, alayered catalyst can promote adhesion to the substrate surface, whiledecreasing carbon nanostructure to nanoparticle adhesion.

Although FIGS. 12 and 13 have depicted carbon nanostructure growth astaking place with basal growth from the catalyst, the skilled artisanwill recognize that other mechanistic forms of carbon nanostructuregrowth are possible. For example, carbon nanostructure growth can alsotake place such that the catalyst resides distal to the growth substrateon the surface of the carbon nanostructure (i.e., tip growth) orsomewhere between tip growth and basal growth. In some embodiments,predominantly basal growth can be selected to aid in carbonnanostructure removal from the growth substrate.

In alternative embodiments, removal of the carbon nanostructure from thegrowth substrate can take place by a process other than fluid shearingor mechanical shearing. In some embodiments, chemical etching can beused to remove the carbon nanostructure from the growth substrate. Insome embodiments, the transition metal nanoparticle catalyst used topromote carbon nanostructure growth can be a transition metal saltcontaining an anion that is selected to etch the growth substrate,thereby facilitating removal of the carbon nanostructure. Suitableetching anions can include, for example, chlorides, sulfates, nitrates,nitrites, and fluorides. In some or other embodiments, a chemical etchcan be employed independently from the catalyst choice. For example,when employing a glass substrate, a hydrogen fluoride etch can be usedto weaken adherence of the carbon nanostructure and/or the transitionmetal nanoparticle catalyst to the substrate.

The carbon nanostructures disclosed herein comprise carbon nanotubes(CNTs) in a network having a complex structural morphology, which hasbeen described in more detail hereinabove. Without being bound by anytheory or mechanism, it is believed that this complex structuralmorphology results from the preparation of the carbon nanostructure on asubstrate under CNT growth conditions that produce a rapid growth rateon the order of several microns per second. The rapid CNT growth rate,coupled with the close proximity of the CNTs to one another, can conferthe observed branching, crosslinking, and shared wall motifs to theCNTs. In the discussion that follows, techniques for producing a carbonnanostructure bound to a fiber substrate are described. For simplicity,the discussion may refer to the carbon nanostructure disposed on thesubstrate interchangeably as CNTs, since CNTs represent the majorstructural component of carbon nanostructures.

In some embodiments, the processes disclosed herein can be applied tonascent fiber materials generated de novo before, or in lieu of,application of a typical sizing solution to the fiber material.Alternatively, the processes disclosed herein can utilize a commercialfiber material, for example, a tow, that already has a sizing applied toits surface. In such embodiments, the sizing can be removed to provide adirect interface between the fiber material and the synthesized carbonnanostructure, although a transition metal nanoparticle catalyst canserve as an intermediate linker between the two. After carbonnanostructure synthesis, further sizing agents can be applied to thefiber material as desired. For the purpose of carbon nanostructureisolation, any of the above mentioned sizing or coatings can be employedto facilitate the isolation process. Equally suitable substrates forforming a carbon nanostructure include tapes, sheets and even threedimensional forms which can be used to provide a shaped carbonnanostructure product. The processes described herein allow for thecontinuous production of CNTs that make up the carbon nanostructurenetwork having uniform length and distribution along spoolable lengthsof tow, tapes, fabrics and other 3D woven structures.

As used herein the term “fiber material” refers to any material whichhas fiber as its elementary structural component. The term encompassesfibers, filaments, yarns, tows, tows, tapes, woven and non-wovenfabrics, plies, mats, and the like.

As used herein the term “spoolable dimensions” refers to fiber materialshaving at least one dimension that is not limited in length, allowingfor the material to be stored on a spool or mandrel. Processes ofdescribed herein can operate readily with 5 to 20 lb. spools, althoughlarger spools are usable. Moreover, a pre-process operation can beincorporated that divides very large spoolable lengths, for example 100lb. or more, into easy to handle dimensions, such as two 50 lb. spools.

As used herein, the term “carbon nanotube” (CNT, plural CNTs) refers toany of a number of cylindrically-shaped allotropes of carbon of thefullerene family including single-walled carbon nanotubes (SWNTs),double-walled carbon nanotubes (DWNTs), multi-walled carbon nanotubes(MWNTs). CNTs can be capped by a fullerene-like structure or open-ended.CNTs include those that encapsulate other materials. CNTs can appear inbranched networks, entangled networks, and combinations thereof. TheCNTs prepared on the substrate within the carbon nanostructure caninclude individual CNT motifs from exclusive MWNTs, SWNTs, or DWNTs, orthe carbon nanostructure can include mixtures of CNT these motifs.

As used herein “uniform in length” refers to an average length of CNTsgrown in a reactor for producing a carbon nanostructure. “Uniformlength” means that the CNTs have lengths with tolerances of plus orminus about 20% of the total CNT length or less, for CNT lengths varyingfrom between about 1 micron to about 500 microns. At very short lengths,such as 1-4 microns, this error may be in a range from between aboutplus or minus 20% of the total CNT length up to about plus or minus 1micron, that is, somewhat more than about 20% of the total CNT length.In the context of the carbon nanostructure, at least one dimension ofthe carbon nanostructure can be controlled by the length of the CNTsgrown.

As used herein “uniform in distribution” refers to the consistency ofdensity of CNTs on a growth substrate, such as a fiber material.“Uniform distribution” means that the CNTs have a density on the fibermaterial with tolerances of plus or minus about 10% coverage defined asthe percentage of the surface area of the fiber covered by CNTs. This isequivalent to +1500 CNTs/μm² for an 8 nm diameter CNT with 5 walls. Sucha figure assumes the space inside the CNTs as fillable.

As used herein, the term “transition metal” refers to any element oralloy of elements in the d-block of the periodic table. The term“transition metal” also includes salt forms of the base transition metalelement such as oxides, carbides, nitrides, and the like.

As used herein, the term “nanoparticle” or NP (plural NPs), orgrammatical equivalents thereof refers to particles sized between about0.1 to about 100 nanometers in equivalent spherical diameter, althoughthe NPs need not be spherical in shape. Transition metal NPs, inparticular, can serve as catalysts for CNT growth on the fibermaterials.

As used herein, the term “sizing agent,” “fiber sizing agent,” or just“sizing,” refers collectively to materials used in the manufacture offibers as a coating to protect the integrity of fibers, provide enhancedinterfacial interactions between a fiber and a matrix material in acomposite, and/or alter and/or enhance particular physical properties ofa fiber.

As used herein, the term “material residence time” refers to the amountof time a discrete point along a fiber material of spoolable dimensionsis exposed to CNT growth conditions during the CNS processes describedherein. This definition includes the residence time when employingmultiple CNT growth chambers.

As used herein, the term “linespeed” refers to the speed at which afiber material of spoolable dimensions is fed through the CNT synthesisprocesses described herein, where linespeed is a velocity determined bydividing CNT chamber(s)' length by the material residence time.

In some embodiments, the CNT-laden fiber material includes a fibermaterial of spoolable dimensions and carbon nanotubes (CNTs) in the formof a carbon nanostructure grown on the fiber material.

Without being bound by any theory or mechanism, transition metal NPs,which serve as a CNT-forming catalyst, can catalyze CNT growth byforming a CNT growth seed structure. In one embodiment, the CNT-formingcatalyst can remain at the base of the fiber material (i.e., basalgrowth). In such a case, the seed structure initially formed by thetransition metal nanoparticle catalyst is sufficient for continuednon-catalyzed seeded CNT growth without allowing the catalyst to movealong the leading edge of CNT growth (i.e., tip growth). In such a case,the NP serves as a point of attachment for the CNS to the fibermaterial.

Compositions having CNS-laden fiber materials are provided in which theCNTs are substantially uniform in length. In the continuous processdescribed herein, the residence time of the fiber material in a CNTgrowth chamber can be modulated to control CNT growth and ultimately,CNT and CNS length. These features provide a means to control specificproperties of the CNTs grown and hence the properties of the CNS. CNTlength can also be controlled through modulation of the carbon feedstockand carrier gas flow rates and reaction temperature. Additional controlof the CNT properties can be obtained by modulating, for example, thesize of the catalyst used to prepare the CNTs. For example, 1 nmtransition metal nanoparticle catalysts can be used to provide SWNTs inparticular. Larger catalysts can be used to prepare predominantly MWNTs.

Additionally, the CNT growth processes employed are useful for providinga CNS-laden fiber material with uniformly distributed CNTs whileavoiding bundling and/or aggregation of the CNTs that can occur inprocesses in which pre-formed CNTs are suspended or dispersed in asolvent medium and applied by hand to the fiber material. In someembodiments, the maximum distribution density, expressed as percentcoverage, that is, the surface area of fiber material that is covered,can be as high as about 55% assuming about 8 nm diameter CNTs with 5walls. This coverage is calculated by considering the space inside theCNTs as being “fillable” space. Various distribution/density values canbe achieved by varying catalyst dispersion on the surface as well ascontrolling gas composition and process speed. Typically for a given setof parameters, a percent coverage within about 10% can be achievedacross a fiber surface. Higher density and shorter CNTs (e.g., less thanabout 100 microns in length) can be useful for improving mechanicalproperties, while longer CNTs (e.g., greater than about 100 microns inlength) with lower density can be useful for improving thermal andelectrical properties, although increased density still can befavorable. A lower density can result when longer CNTs are grown. Thiscan be the result of the higher temperatures and more rapid growthcausing lower catalyst particle yields.

CNS-laden fiber materials can include a fiber material such asfilaments, a fiber yarn, a fiber tow, a fiber-braid, a woven fabric, anon-woven fiber mat, a fiber ply, and other 3D woven structures.Filaments include high aspect ratio fibers having diameters ranging insize from between about 1 micron to about 100 microns. Fiber tows aregenerally compactly associated bundles of filaments and are usuallytwisted together to give yarns.

Yarns include closely associated bundles of twisted filaments. Eachfilament diameter in a yarn is relatively uniform. Yarns have varyingweights described by their ‘tex,’ expressed as weight in grams of 1000linear meters, or denier, expressed as weight in pounds of 10,000 yards,with a typical tex range usually being between about 200 tex to about2000 tex.

Tows include loosely associated bundles of untwisted filaments. As inyarns, filament diameter in a tow is generally uniform. Tows also havevarying weights and the tex range is usually between 200 tex and 2000tex. They are frequently characterized by the number of thousands offilaments in the tow, for example 12K tow, 24K tow, 48K tow, and thelike.

Tapes are materials that can be assembled as weaves or can representnon-woven flattened tows. Tapes can vary in width and are generallytwo-sided structures similar to ribbon. CNT infusion can take place onone or both sides of a tape. CNS-laden tapes can resemble a “carpet” or“forest” on a flat substrate surface. However, the CNS can be readilydistinguished from conventional aligned CNT forests due to thesignificantly higher degree of branching and crosslinking that occurs inthe CNS structural morphology. Again, processes described herein can beperformed in a continuous mode to functionalize spools of tape.

Fiber braids represent rope-like structures of densely packed fibers.Such structures can be assembled from yarns, for example. Braidedstructures can include a hollow portion or a braided structure can beassembled about another core material.

CNTs lend their characteristic properties such as mechanical strength,low to moderate electrical resistivity, high thermal conductivity, andthe like to the CNS-laden fiber material. For example, in someembodiments, the electrical resistivity of a carbon nanotube-laden fibermaterial is lower than the electrical resistivity of a parent fibermaterial. Likewise, such properties can translate to the isolated CNS.More generally, the extent to which the resulting CNS-laden fiberexpresses these characteristics can be a function of the extent anddensity of coverage of the fiber by the carbon nanotubes. Any amount ofthe fiber surface area, from 0-55% of the fiber can be covered assumingan 8 nm diameter, 5-walled MWNT (again this calculation counts the spaceinside the CNTs as fillable). This number is lower for smaller diameterCNTs and more for greater diameter CNTs. 55% surface area coverage isequivalent to about 15,000 CNTs/micron². Further CNT properties can beimparted to the fiber material in a manner dependent on CNT length, asdescribed above. CNTs within the carbon nanostructure can vary in lengthfrom between about 1 micron to about 500 microns, including about 1micron, about 2 microns, about 3 microns, about 4 micron, about 5,microns, about 6, microns, about 7 microns, about 8 microns, about 9microns, about 10 microns, about 15 microns, about 20 microns, about 25microns, about 30 microns, about 35 microns, about 40 microns, about 45microns, about 50 microns, about 60 microns, about 70 microns, about 80microns, about 90 microns, about 100 microns, about 150 microns, about200 microns, about 250 microns, about 300 microns, about 350 microns,about 400 microns, about 450 microns, about 500 microns, and all valuesand sub-ranges in between. CNTs can also be less than about 1 micron inlength, including about 0.5 microns, for example. CNTs can also begreater than 500 microns, including for example, about 510 microns,about 520 microns, about 550 microns, about 600 microns, about 700microns and all values and subranges in between. It will be understoodthat such lengths accommodate the presence of crosslinking and branchingand therefore the length may be the composite length measured from thebase of the growth substrate up to the edges of the CNS.

CNSs described herein can also incorporate CNTs have a length from about1 micron to about 10 microns. Such CNT lengths can be useful inapplication to increase shear strength. CNTs can also have a length fromabout 5 to about 70 microns. Such CNT lengths can be useful inapplications for increased tensile strength if the CNTs are aligned inthe fiber direction. CNTs can also have a length from about 10 micronsto about 100 microns. Such CNT lengths can be useful to increaseelectrical/thermal properties as well as mechanical properties. CNTshaving a length from about 100 microns to about 500 microns can also bebeneficial to increase electrical and thermal properties. Such controlof CNT length is readily achieved through modulation of carbon feedstockand inert gas flow rates coupled with varying linespeeds and growthtemperatures.

In some embodiments, compositions that include spoolable lengths ofCNS-laden fiber materials can have various uniform regions withdifferent lengths of CNTs. For example, it can be desirable to have afirst portion of CNS-laden fiber material with uniformly shorter CNTlengths to enhance shear strength properties, and a second portion ofthe same spoolable material with a uniform longer CNT length to enhanceelectrical or thermal properties.

Processes for rapid CNS growth on fiber materials allow for control ofthe CNT lengths with uniformity in continuous processes with spoolablefiber materials. With material residence times between 5 to 300 seconds,linespeeds in a continuous process for a system that is 3 feet long canbe in a range anywhere from about 0.5 ft/min to about 36 ft/min andgreater. The speed selected depends on various parameters as explainedfurther below.

In some embodiments, a material residence time of about 5 seconds toabout 30 seconds can produce CNTs having a length between about 1 micronto about 10 microns. In some embodiments, a material residence time ofabout 30 seconds to about 180 seconds can produce CNTs having a lengthbetween about 10 microns to about 100 microns. In still furtherembodiments, a material residence time of about 180 seconds to about 300seconds can produce CNTs having a length between about 100 microns toabout 500 microns. One skilled in the art will recognize that theseranges are approximate and that CNT length can also be modulated byreaction temperatures, and carrier and carbon feedstock concentrationsand flow rates.

In some embodiments, continuous processes for CNS growth can include (a)disposing a carbon nanotube-forming catalyst on a surface of a fibermaterial of spoolable dimensions; and (b) synthesizing carbon nanotubesdirectly on the fiber material, thereby forming a CNS-laden fibermaterial. For a 9 foot long system, the linespeed of the process canrange from between about 1.5 ft/min to about 108 ft/min. The linespeedsachieved by the process described herein allow the formation ofcommercially relevant quantities of CNS-laden fiber materials with shortproduction times. For example, at 36 ft/min linespeed, the quantities ofCNS-laden fibers (over 5% CNTs on fiber by weight) can exceed over 100pound or more of material produced per day in a system that is designedto simultaneously process 5 separate tows (20 lb/tow). Systems can bemade to produce more tows at once or at faster speeds by repeatinggrowth zones.

As described further below the catalyst can be prepared as a liquidsolution that contains CNT-forming catalyst that contains transitionmetal nanoparticles. The diameters of the synthesized nanotubes arerelated to the size of the transition metal nanoparticles as describedabove. In some embodiments, commercial dispersions of CNT-formingtransition metal nanoparticle catalysts are available and can be usedwithout dilution, and in other embodiments commercial dispersions ofcatalyst can be diluted. Whether to dilute such solutions can depend onthe desired density and length of CNT to be grown as described above.

Carbon nanotube synthesis can be based on a chemical vapor deposition(CVD) process and occurs at elevated temperatures. The specifictemperature is a function of catalyst choice, but will typically be in arange of about 500° C. to about 1000° C. This operation involves heatingthe fiber material to a temperature in the aforementioned range tosupport carbon nanotube synthesis.

CVD-promoted nanotube growth on the catalyst-laden fiber material isthen performed. The CVD process can be promoted by, for example, acarbon-containing feedstock gas such as acetylene, ethylene, methane,and/or propane. The CNT synthesis processes generally use an inert gas(nitrogen, argon, helium) as a primary carrier gas. The carbon feedstockis generally provided in a range from between about 0% to about 50% ofthe total mixture. A substantially inert environment for CVD growth isprepared by removal of moisture and oxygen from the growth chamber.

The operation of disposing a catalyst on the fiber material can beaccomplished by spraying or dip coating a solution or by gas phasedeposition via, for example, a plasma process. Thus, in someembodiments, after forming a solution of a catalyst in a solvent,catalyst can be applied by spraying or dip coating the fiber materialwith the solution, or combinations of spraying and dip coating. Eithertechnique, used alone or in combination, can be employed once, twice,thrice, four times, up to any number of times to provide a fibermaterial that is sufficiently uniformly coated with CNT-formingcatalyst. When dip coating is employed, for example, a fiber materialcan be placed in a first dip bath for a first residence time in thefirst dip bath. When employing a second dip bath, the fiber material canbe placed in the second dip bath for a second residence time. Forexample, fiber materials can be subjected to a solution of CNT-formingcatalyst for between about 3 seconds to about 90 seconds depending onthe dip configuration and linespeed. Employing spraying or dip coatingprocesses, a fiber material with a surface density of catalyst of lessthan about 5% surface coverage to as high as about 80% coverage, inwhich the CNT-forming catalyst nanoparticles are nearly monolayer. Insome embodiments, the process of coating the CNT-forming catalyst on thefiber material should produce no more than a monolayer. For example, CNTgrowth on a stack of CNT-forming catalyst can erode the degree ofinfusion of the CNT to the fiber material. In other embodiments, thetransition metal catalyst can be deposited on the fiber material usingevaporation techniques, electrolytic deposition techniques, and otherdeposition processes, such as addition of the transition metal catalystto a plasma feedstock gas as a metal organic, metal salt or othercomposition promoting gas phase transport.

Because processes for growing carbon nanostructures are designed to becontinuous, a spoolable fiber material can be dip-coated in a series ofbaths where dip coating baths are spatially separated. In continuousprocesses in which nascent fibers are being generated de novo, dip bathor spraying of CNT-forming catalyst can be the first step. In otherembodiments, the CNT-forming catalyst can be applied to newly formedfibers in the presence of other sizing agents. Such simultaneousapplication of CNT-forming catalyst and other sizing agents can providethe CNT-forming catalyst in the surface of the sizing on the fibermaterial to create a poorly adhered CNT coating.

The catalyst solution employed can be a transition metal nanoparticlewhich can be any d-block transition metal, as described above. Inaddition, the nanoparticles can include alloys and non-alloy mixtures ofd-block metals in elemental form or in salt form, and mixtures thereof.Such salt forms include, without limitation, oxides, carbides, acetates,and nitrides. Non-limiting exemplary transition metal NPs include Ni,Fe, Co, Mo, Cu, Pt, Au, and Ag and salts thereof and mixtures thereof.In some embodiments, such CNT-forming catalysts are disposed on thefiber by applying or infusing a CNT-forming catalyst directly to thefiber material simultaneously with barrier coating deposition. Many ofthese transition metal catalysts are readily commercially available froma variety of suppliers, including, for example, Sigma Aldrich (St.Louis, Mo.) or Ferrotec Corporation (Bedford, N.H.)

Catalyst solutions used for applying the CNT-forming catalyst to thefiber material can be in any common solvent that allows the CNT-formingcatalyst to be uniformly dispersed throughout. Such solvents caninclude, without limitation, water, acetone, hexane, isopropyl alcohol,toluene, ethanol, methanol, tetrahydrofuran (THF), cyclohexane or anyother solvent with controlled polarity to create an appropriatedispersion of the CNT-forming catalyst nanoparticles. Concentrations ofCNT-forming catalyst can be in a range from about 1:1 to 1:10000catalyst to solvent. Such concentrations can be used when the barriercoating and CNT-forming catalyst are applied simultaneously as well.

In some embodiments heating of the fiber material can be at atemperature that is between about 500° C. and about 1000° C. tosynthesize carbon nanotubes after deposition of the CNT-formingcatalyst. Heating at these temperatures can be performed prior to orsubstantially simultaneously with introduction of a carbon feedstock forCNT growth.

In some embodiments, the processes for producing a carbon nanostructureinclude removing a sizing agent from a fiber material, applying anadhesion-inhibiting coating (i.e., an anti-adhesive coating) conformallyover the fiber material, applying a CNT-forming catalyst to the fibermaterial, heating the fiber material to at least 500° C., andsynthesizing carbon nanotubes on the fiber material. In someembodiments, operations of the CNS-growth process can include removingsizing from a fiber material, applying an adhesion-inhibiting coating tothe fiber material, applying a CNT-forming catalyst to the fiber,heating the fiber to CNT-synthesis temperature and performingCVD-promoted CNS growth on the catalyst-laden fiber material. Thus,where commercial fiber materials are employed, processes forconstructing CNS-laden fibers can include a discrete step of removingsizing from the fiber material before disposing adhesion-inhibitingcoating and the catalyst on the fiber material.

Synthesizing carbon nanotubes on the fiber material can include numeroustechniques for forming carbon nanotubes, including those disclosed inco-pending U.S. Patent Application Publication No. 2004/0245088, whichis incorporated herein by reference. The CNS grown on the fibers can beformed by techniques such as, for example, micro-cavity, thermal orplasma-enhanced CVD techniques, laser ablation, arc discharge, and highpressure carbon monoxide (HiPCO). In some embodiments, any conventionalsizing agents can be removed prior CNT synthesis. In some embodiments,acetylene gas can be ionized to create a jet of cold carbon plasma forCNT synthesis. The plasma is directed toward the catalyst-bearing fibermaterial. Thus, in some embodiments for synthesizing CNS on a fibermaterial include (a) forming a carbon plasma; and (b) directing thecarbon plasma onto the catalyst disposed on the fiber material. Thediameters of the CNTs that are grown are dictated by the size of theCNT-forming catalyst as described above. In some embodiments, the sizedfiber material is heated to between about 550° C. to about 800° C. tofacilitate CNS synthesis. To initiate the growth of CNTs, two gases arebled into the reactor: a process gas such as argon, helium, or nitrogen,and a carbon-containing gas, such as acetylene, ethylene, ethanol ormethane. CNTs grow at the sites of the CNT-forming catalyst.

In some embodiments, the CVD growth is plasma-enhanced. A plasma can begenerated by providing an electric field during the growth process. CNTsgrown under these conditions can follow the direction of the electricfield. Thus, by adjusting the geometry of the reactor, verticallyaligned carbon nanotubes can be grown radially about a cylindricalfiber. In some embodiments, a plasma is not required for radial growthabout the fiber. For fiber materials that have distinct sides such astapes, mats, fabrics, plies, and the like, catalyst can be disposed onone or both sides and correspondingly, CNTs can be grown on one or bothsides as well.

As described above, CNS-synthesis can be performed at a rate sufficientto provide a continuous process for functionalizing spoolable fibermaterials. Numerous apparatus configurations facilitate such continuoussynthesis and result in the complex CNS morphology, as exemplifiedbelow.

One configuration for continuous CNS synthesis involves an optimallyshaped (shaped to match the size and shape of the substrate) reactor forthe synthesis and growth of carbon nanotubes directly on fibermaterials. The reactor can be designed for use in a continuous in-lineprocess for producing CNS-bearing fibers. In some embodiments, CNSs canbe grown via a chemical vapor deposition (“CVD”) process at atmosphericpressure and at elevated temperature in the range of about 550° C. toabout 800° C. in a multi-zone reactor. The fact that the synthesisoccurs at atmospheric pressure is one factor that facilitates theincorporation of the reactor into a continuous processing line forCNS-on-fiber synthesis. Another advantage consistent with in-linecontinuous processing using such a zoned reactor is that CNT growthoccurs in a seconds, as opposed to minutes (or longer) as in otherprocedures and apparatus configurations typical in the art.

CNS synthesis reactors in accordance with the various embodimentsinclude the following features:

Optimally Shaped Synthesis Reactors: Adjusting the size of the growthchamber to more effectively match the size of the substrate travelingthrough it improves reaction rates as well as process efficiency byreducing the overall volume of the reaction vessel. The cross section ofthe optimally shaped growth chamber can be maintained below a volumeratio of chamber to substrate of 10,000. In some embodiments, the crosssection of the chamber is maintained at a volume ratio of below 1,000.In other embodiments, the cross section of the chamber is maintained ata volume ratio below 500.

Although gas deposition processes, such as CVD, are typically governedby pressure and temperature alone, volume has a significant impact onthe efficiency of deposition. By matching the shape of the substratewith the growth chamber there is greater opportunity for productive CNSforming reactions to occur. It should be appreciated that in someembodiments, the synthesis reactor has a cross section that is describedby polygonal forms according the shape of the substrate upon which theCNS is grown to provide a reduction in reactor volume. In someembodiments, gas can be introduced at the center of the reactor orwithin a target growth zone, symmetrically, either through the sides orthrough the top and bottom plates of the reactor. This improves theoverall CNT growth rate because the incoming feedstock gas iscontinuously replenishing at the hottest portion of the system, which iswhere CNT growth is most active. This constant gas replenishment is animportant aspect to the increased growth rate exhibited by the shapedCNT reactors.

Zoning: Chambers that provide a relatively cool purge zone depend fromboth ends of the synthesis reactor. Applicants have determined that ifhot gas were to mix with the external environment (i.e., outside of thereactor), there would be an increase in degradation of most fibermaterials. The cool purge zones provide a buffer between the internalsystem and external environments. Typical CNT synthesis reactorconfigurations known in the art typically require that the substrate iscarefully (and slowly) cooled. The cool purge zone at the exit of thepresent CNS growth reactor achieves the cooling in a short period oftime, as required for the continuous in-line processing.

Non-contact, hot-walled, metallic reactor: In some embodiments, ahot-walled reactor made of metal can be employed, in particularstainless steel. This may appear counterintuitive because metal, andstainless steel in particular, is more susceptible to carbon deposition(i.e., soot and by-product formation). Thus, most CNT reactorconfigurations use quartz reactors because there is less carbondeposited, quartz is easier to clean, and quartz facilitates sampleobservation.

However, it has been observed that the increased soot and carbondeposition on stainless steel results in more consistent, faster, moreefficient, and more stable CNT growth. Without being bound by theory ithas been indicated that, in conjunction with atmospheric operation, theCVD process occurring in the reactor is diffusion limited. That is, thecatalyst is “overfed;” too much carbon is available in the reactorsystem due to its relatively higher partial pressure (than if thereactor was operating under partial vacuum). As a consequence, in anopen system-especially a clean one-too much carbon can adhere tocatalyst particles, compromising their ability to synthesize CNTs. Insome embodiments, the rectangular reactor is intentionally run when thereactor is “dirty,” that is with soot deposited on the metallic reactorwalls. Once carbon deposits to a monolayer on the walls of the reactor,carbon will readily deposit over itself. Since some of the availablecarbon is “withdrawn” due to this mechanism, the remaining carbonfeedstock, in the form of radicals, react with the catalyst at a ratethat does not poison the catalyst. Existing systems run “cleanly” which,if they were open for continuous processing, would produce a much loweryield of CNTs at reduced growth rates.

Although it is generally beneficial to perform CNT synthesis “dirty” asdescribed above, certain portions of the apparatus, such as gasmanifolds and inlets, can nonetheless negatively impact the CNT growthprocess when soot created blockages. In order to combat this problem,such areas of the CNT growth reaction chamber can be protected with sootinhibiting coatings such as silica, alumina, or MgO. In practice, theseportions of the apparatus can be dip-coated in these soot inhibitingcoatings. Metals such as INVAR® can be used with these coatings as INVARhas a similar CTE (coefficient of thermal expansion) ensuring properadhesion of the coating at higher temperatures, preventing the soot fromsignificantly building up in critical zones.

In some embodiments, the reaction chamber may comprise SiC, alumina, orquartz as the primary chamber materials because they do not react withthe reactive gases of CNS synthesis. This feature allows for increasedefficiency and improves operability over long durations of operation.

Combined Catalyst Reduction and CNS Synthesis. In the CNT synthesisreactor, both catalyst reduction and CNS growth can occur within thereactor. This feature is significant because the reduction operationcannot be accomplished timely enough for use in a continuous process ifperformed as a discrete operation. In typical carbon nanotube synthesisprocesses, catalyst reduction typically takes 1-12 hours to perform. Insynthesizing a carbon nanostructure according to the embodimentsdescribed herein, both catalyst reduction and CNS synthesis occur in thereactor, at least in part, due to the fact that carbon feedstock gas isintroduced at the center of the reactor, not the end as would typicallybe performed using cylindrical reactors. The reduction process occurs asthe fibers enter the heated zone; by this point, the gas has had time toreact with the walls and cool off prior to reacting with the catalystand causing the oxidation-reduction (via hydrogen radical interactions).It is this transition region where the reduction occurs. At the hottestisothermal zone in the system, the CNS growth occurs, with the greatestgrowth rate occurring proximal to the gas inlets near the center of thereactor.

In some embodiments, when loosely affiliated fiber materials, such astow are employed, the continuous process can include operations thatspreads out the strands and/or filaments of the tow. Thus, as a tow isunspooled it can be spread using a vacuum-based fiber spreading system,for example. When employing sized fibers, which can be relatively stiff,additional heating can be employed in order to “soften” the tow tofacilitate fiber spreading. The spread fibers which comprise individualfilaments can be spread apart sufficiently to expose an entire surfacearea of the filaments, thus allowing the tow to more efficiently reactin subsequent process steps. Such spreading can approach between about 4inches to about 6 inches across for a 3 k tow. The spread tow can passthrough a surface treatment step that is composed of a plasma system asdescribed above. After a barrier coating is applied and roughened,spread fibers then can pass through a CNT-forming catalyst dip bath. Theresult is fibers of the tow that have catalyst particles distributedradially on their surface. The catalyzed-laden fibers of the tow thenenter an appropriate CNT growth chamber, such as the optimally shapedchamber described above, where a flow through atmospheric pressure CVDor PE-CVD process is used to synthesize the CNS at rates as high asseveral microns per second. The fibers of the tow, now with radiallyaligned CNTs in the form of the CNS morphology, exit the CNT growthreactor.

In some embodiments. CNS-laden fiber materials can pass through yetanother treatment process prior to isolation that, in some embodimentsis a plasma process used to functionalize the CNS. Additionalfunctionalization of CNS can be used to promote their adhesion toparticular resins. Thus, in some embodiments, the processes can provideCNS-laden fiber materials having functionalized CNS. Completing thisfunctionalization process while the CNS are still on the fiber canimprove treatment uniformity.

In some embodiments, a continuous process for growing of CNS onspoolable fiber materials can achieve a linespeed between about 0.5ft/min to about 36 ft/min. In this embodiment where the CNT growthchamber is 3 feet long and operating at a 750° C. growth temperature,the process can be run with a linespeed of about 6 ft/min to about 36ft/min to produce, for example, CNTs having a length between about 1micron to about 10 microns. The process can also be run with a linespeedof about 1 ft/min to about 6 ft/min to produce, for example, CNTs havinga length between about 10 microns to about 100 microns. The process canbe run with a linespeed of about 0.5 ft/min to about 1 ft/min toproduce, for example, CNTs having a length between about 100 microns toabout 200 microns. The CNT length is not tied only to linespeed andgrowth temperature, however, the flow rate of both the carbon feedstockand the inert carrier gases can also influence CNT length. For example,a flow rate consisting of less than 1% carbon feedstock in inert gas athigh linespeeds (6 ft/min to 36 ft/min) will result in CNTs having alength between 1 micron to about 5 microns. A flow rate consisting ofmore than 1% carbon feedstock in inert gas at high linespeeds (6 ft/minto 36 ft/min) will result in CNTs having length between 5 microns toabout 10 microns.

In some embodiments, more than one material can be run simultaneouslythrough the process. For example, multiple tapes tows, filaments, strandand the like can be run through the process in parallel. Thus, anynumber of pre-fabricated spools of fiber material can be run in parallelthrough the process and re-spooled at the end of the process. The numberof spooled fiber materials that can be run in parallel can include one,two, three, four, five, six, up to any number that can be accommodatedby the width of the CNT-growth reaction chamber. Moreover, when multiplefiber materials are run through the process, the number of collectionspools can be less than the number of spools at the start of theprocess. In such embodiments, strands, tows, or the like can be sentthrough a further process of combining such fiber materials into higherordered fiber materials such as woven fabrics or the like. Thecontinuous process can also incorporate a post processing chopper thatfacilitates the formation CNS-laden chopped fiber mats, for example.

The continuous processing can optionally include further CNS chemistry.Because the CNS is a polymeric network of CNTs, all the chemistriesassociated with individualized CNTs may be carried out on the CNSmaterials. Such chemistries can be performed inline with CNS preparationor separately. In some embodiments, the CNS can be modified while it isstill substrate-bound. This can aid in purification of the CNS material.In other embodiments, the CNS chemistry can be performed after it isremoved from the substrate upon which it was synthesized. Exemplarychemistries include those described herein above in addition tofluorination, oxidation, reduction, and the like. In some embodiments,the CNS material can be used to store hydrogen. In some embodiments, theCNS structure can be modified by attachment to another polymericstructure to form a diblock polymer. In some embodiments, the CNSstructure can be used as a platform for attachment of a biomolecule. Insome embodiments, the CNS structure can be configured to be used as asensor. In some embodiments, the CNS structure can be incorporated in amatrix material to form a composite material. In some embodiments, a CNSstructure can be modified with reagents known to unzip CNTs and formgraphene nanoribbons. Numerous other chemistries and downstreamapplications can be recognized by those skilled in the art.

In some embodiments, the processes allow for synthesizing a first amountof a first type of CNS on the fiber material, in which the first type ofCNS comprises CNTs selected to alter at least one first property of thefiber material. Subsequently, the processes allow for synthesizing asecond amount of a second type of CNS on the fiber material, in whichthe second type of CNS contains carbon nanotubes selected to alter atleast one second property of the fiber material.

In some embodiments, the first amount and second amount of CNTs aredifferent. This can be accompanied by a change in the CNT type or not.Thus, varying the density of CNS can be used to alter the properties ofthe original fiber material, even if the CNT type remains unchanged. CNTtype can include CNT length and the number of walls, for example. Insome embodiments the first amount and the second amount are the same. Ifdifferent properties are desirable along two different stretches of thefiber material, then the CNT type can be changed, such as the CNTlength. For example, longer CNTs can be useful in electrical/thermalapplications, while shorter CNTs can be useful in mechanicalstrengthening applications.

Electrical conductivity or specific conductance is a measure of amaterial's ability to conduct an electric current. CNTs with particularstructural parameters such as the degree of twist, which relates to CNTchirality, can be highly conducting, thus exhibiting metallicproperties. A recognized system of nomenclature for CNT chirality hasbeen formalized and is recognized by those skilled in the art. Thus, forexample, CNTs are distinguished from each other by a double index (n,m)where n and m are integers that describe the cut and wrapping ofhexagonal graphite so that it makes a tube when it is wrapped onto thesurface of a cylinder and the edges are sealed together. When the twoindices are the same, m=n, the resultant tube is said to be of the“arm-chair” (or n,n) type, since when the tube is cut perpendicular tothe CNT axis only the sides of the hexagons are exposed and theirpattern around the periphery of the tube edge resembles the arm and seatof an arm chair repeated n times. Arm-chair CNTs, in particular SWNTs,are metallic, and have extremely high electrical and thermalconductivity. In addition, such SWNTs have extremely high tensilestrength.

In addition to the degree of twist, CNT diameter also effects electricalconductivity. As described above, CNT diameter can be controlled by useof controlled size CNT-forming catalyst nanoparticles. CNTs can also beformed as semi-conducting materials. Conductivity in multi-walled CNTs(MWNTs) can be more complex. Interwall reactions within MWNTs canredistribute current over individual tubes non-uniformly. By contrast,there is no change in current across different parts of metallicsingle-walled nanotubes (SWNTs). Carbon nanotubes also have very highthermal conductivity, comparable to diamond crystal and in-planegraphite sheets. Any of these characteristic properties of CNTs can beexhibited in a CNS. In some embodiments, the CNS can facilitaterealization of property enhancements in materials in which the CNS isincorporated to a degree that is greater than that of individualizedCNTs.

Although the invention has been described with reference to thedisclosed embodiments, those skilled in the art will readily appreciatethat these are only illustrative of the invention. It should beunderstood that various modifications can be made without departing fromthe spirit of the invention. The invention can be modified toincorporate any number of variations, alterations, substitutions orequivalent arrangements not heretofore described, but which arecommensurate with the spirit and scope of the invention. Additionally,while various embodiments of the invention have been described, it is tobe understood that aspects of the invention may include only some of thedescribed embodiments. Accordingly, the invention is not to be seen aslimited by the foregoing description.

What is claimed is the following:
 1. A three-dimensional printing method comprising: depositing a printing composition in a layer-by-layer deposition process; wherein the printing composition comprises a solidifiable matrix and a plurality of carbon nanostructures dispersed in the solidifiable matrix, the carbon nanostructures comprising a plurality of carbon nanotubes that are branched, crosslinked, and share common walls with one another, and while depositing the printing composition, applying a focused input of microwave radiation in proximity to a location where the printing composition is being deposited, thereby heating the carbon nanostructures at the location and promoting consolidation of the printing composition within an object being produced by the layer-by-layer deposition process.
 2. The method of claim 1, wherein the location is a previously deposited layer of the object, heating of the carbon nanostructures in the previously deposited layer promoting consolidation of the solidifiable matrix therein with the solidifiable matrix in a subsequently deposited layer of the printing composition.
 3. The method of claim 2, wherein the focused input of microwave radiation is applied at the location where the printing composition is being deposited.
 4. The three-dimensional printing method of claim 1, wherein the printing composition is deposited as a plurality of droplets.
 5. The three-dimensional printing method of claim 1, wherein the carbon nanostructures are free of a growth substrate and are in the form of a flake material.
 6. The three-dimensional printing method of claim 1, wherein the focused input of microwave radiation has a frequency such that interior portions of the object remain substantially unheated by the microwave radiation.
 7. The three-dimensional printing method of claim 1, wherein the microwave radiation has a frequency of about 10 GHz or higher.
 8. The three-dimensional printing method of claim 1, wherein the solidifiable matrix comprises a thermoplastic polymer.
 9. The three-dimensional printing method of claim 1, wherein the focused input of microwave radiation is supplied from a microwave transmission line located upon the same side of the object from which the printing composition is being deposited, the microwave transmission line exhibiting a standing wave therein; wherein the standing wave contacts the object at one or more locations and heats the carbon nanostructures therein.
 10. The three-dimensional printing method of claim 9, wherein the standing wave tangentially contacts a previously deposited layer of the object.
 11. The three-dimensional printing method of claim 9, wherein the standing wave contacts the object at a single location.
 12. The three-dimensional printing method of claim 9, wherein the standing wave contacts a previously deposited layer of the object at the location where the printing composition is being deposited.
 13. The three-dimensional printing method of claim 9, wherein the printing composition is deposited from a print head that is coupled to the microwave transmission line.
 14. A three-dimensional printing method comprising: depositing a printing composition in a layer-by-layer deposition process; wherein the printing composition comprises a solidifiable matrix and a plurality of carbon nanostructures dispersed in the solidifiable matrix, the carbon nanostructures comprising a plurality of carbon nanotubes that are branched, crosslinked, and share common walls with one another, and wherein the printing composition is deposited from a print head that is coupled to a microwave transmission line; establishing a standing wave in the microwave transmission line; and while depositing the printing composition, contacting the standing wave at one more locations with a previously deposited layer of an object being produced by the layer-by-layer deposition process, thereby heating the carbon nanostructures at the one or more locations and promoting consolidation of the printing composition within the object.
 15. The method of claim 14, wherein the standing wave contacts the previously deposited layer of the object at a location where the printing composition is being deposited.
 16. The method of claim 14, wherein the carbon nanostructures are free of a growth substrate and are in the form of a flake material.
 17. The method of claim 14, wherein the microwave radiation has a frequency of about 10 GHz or higher.
 18. The method of claim 14, wherein the solidifiable matrix comprises a thermoplastic polymer.
 19. The method of claim 14, wherein the standing wave contacts the object at a single location.
 20. A three-dimensional printing system comprising: a print head; a reservoir of a printing composition that is fluidly coupled to the print head, the printing composition comprising a solidifiable matrix and a plurality of carbon nanostructures dispersed in the solidifiable matrix, the carbon nanostructures comprising a plurality of carbon nanotubes that are branched, crosslinked, and share common walls with one another; a stage configured for deposition thereon of the printing composition from the print head; wherein the print head or the stage is movable and under control of a computer; and a microwave emitter configured to apply a focused input of microwave radiation in proximity to a location where the printing composition is being deposited from the print head.
 21. The three-dimensional printing system of claim 20, wherein the microwave emitter comprises a microwave transmission line that is coupled to the print head, the microwave transmission line being configured to establish a standing wave therein.
 22. The three-dimensional printing system of claim 21, further comprising: a reflective load located at a terminus of the microwave transmission line. 